The Urban Dictionary Mug

A substance made up of about 60% amphetamine.
In contrast to speed, which is made up of about 5% amphetamine.

Often methamphetamine is defined as base: "a purer form of speed".
Methamphetamine is a completely different chemical, with different effects. Base is the same chemical as speed, just in much higher concentrations.
Usually has a different effect to speed, as the higher dosage doesn't just mean fuller effects, but whole new ones as well.

NB: both amphetamine and methamphetamine have absolutely NOTHING to do with 2,3 methylenedioxymethamphetamine (ecstasy), despite the uneducated and ill-informed ramblings of the DEA.

personone: Meth is just a purer form of speed.
persontwo: No, that's base you idiot. Meth is a different chemical to speed.
personone: Allow me to kiss your educated feet, master. I revere in your mighty intelligence.